Two teams behind discovery of top quark

March 17, 1995

The March 2 announcement from Fermilab that the top quark has been discovered was the culmination of a search that began 18 years ago and eventually involved nearly a thousand scientists from more than a dozen nations.

Two experimental collaborations, of approximately equal size, working independently and in fierce competition at Fermilab's Tevatron--the world's most powerful proton-antiproton collider--produced consistent observations that established the production and decay of top quarks. LBL scientists have been prominently involved in both experiments from their inception and played key roles in last week's landmark discovery.

The older of the two groups, which began its preparations in 1980, designed and built the Collider Detector Facility (CDF) to conduct its experiments. The second group, which began its work in 1984, designed and built the detector array known as D-Zero (D0) to conduct experiments. Over the years, dozens of LBL researchers have worked on each. Currently, the leader of LBL's CDF group is Lina Galtieri and the leader of the D0 group is Ron Madaras. Both are senior staff physicists in the Physics Division (PD). Another PD physicist, Bill Carithers, is a visiting scientist at Fermilab, where he serves as co-spokesperson for the CDF collaboration.

The CDF consists of three main components: a central detector system in the middle of which collisions between protons and antiprotons take place, plus a forward and backward detector to catch particles at small angles. The central detector system weighs about 2,000 tons and features several types of calorimeters for measuring energies, a Time Projection Chamber, a powerful superconducting magnet, and large drift chambers. At the start of the project, LBL was responsible for the hadronic component of the end-cap calorimeters--the component that measured the energies of hadrons, subatomic particles that interact through the strong force. The LBL end-cap hadron calorimeters were completed and shipped to in 1985 Fermilab, where they were combined with the rest of CDF's calorimeters to measure the total energy released when a proton and antiproton collided. This information is necessary to select events with the characteristic energy signature of top quark production.

More recently, physicists and engineers at LBL designed a sophisticated microchip for the Silicon Vertex Detector (SVX), an extremely high resolution instrument in the central CDF detector system that enables precise identification and tracking of particle trajectories. There are 500,000 particle collisions per second occurring at the center of the CDF and the SVX is able to analyze every one of them thanks to its innovative integrated circuit design and readout electronics, which were developed at LBL.

"The microchip allows us to record the signals left by the particles that traverse the four layers of silicon detectors in the SVX," says Galtieri. "This allows us to reconstruct the point where the particles originated. For particles which are decay products of b quarks, the origin is found to be displaced from the point where the proton and antiproton have collided. The presence of b quarks in conjunction with electrons from the W boson decay is a powerful signature for the existence of top quark production."

Galtieri's group at LBL also developed a method to determine the mass of the top quark, which they found to be approximately the same as an atom of gold. This makes the top quark by far the heaviest elementary particle ever observed. This analysis was based on a technique developed at LBL in the 1960s for bubble chamber studies by the research group led by the late Nobel laureate Luis Alvarez. Galtieri's group was able to extend the technique to handle much more complex processes than it was originally designed for, including the production of top quarks. Prior to this, many scientists did not believe it possible to measure mass in such a complex process.

Last April, the CDF group announced the first evidence of the top quark. At that time, CDF spokespersons said they had a strong case for the top quark but lacked enough data to claim discovery. Since then, both the CDF and the D0 groups have tripled the amount of data collected and the threshold set by particle physicists for discovery has been reached.

The D0 detector system consists of a compact tracking detector system, a hermetic calorimeter, and a large solid angle muon detector system. The innermost component of the central tracking detector system is the Vertex Chamber, which was designed, built, tested, and commissioned at LBL. Tracking detectors supplement calorimeters by measuring particle trajectories. Only when trajectory and energy measurements are combined can scientists identify and characterize particles. LBL's Vertex Chamber contains thousands of fine wires, charged with different voltages, that can be used to locate the vertex of an event--the precise spot within the beam pipe where a proton and an antiproton collided. The chamber is divided into three independent layers, each of which can measure the azimuth and longitudinal position of the tracks left by a charged particle.

LBL was also responsible for designing, fabricating, and testing the D0 uranium liquid-argon End Cap Electromagnetic (ECEM) calorimeters, which are used for the precise energy and position measurement of forward electrons and photons.

"These are two of the most demanding pieces of apparatus in the D0 Detector," says Madaras. "Each ECEM is built as a single unit of monolithic multilayer printed circuit signal board disks and uranium absorber disks with special printed circuit boards that are used to read out 7,500 signals."

Like their CDF counterparts, the D0 collaboration also measured the mass of the top quark and found it to be about 200 times the mass of a proton, slightly larger than the CDF's measurement but consistent when the margins of error are taken into consideration. For their mass measurements, the D0 group developed its own multidimensional analysis of the patterns of top quark and W boson decay.

The D0 group at LBL had a significant role in the development of the mass analysis and, in addition, developed its own analysis of the shape of the energy distributions in top quark and W boson decay," says physicist Tom Trippe, deputy leader of the LBL D0 group. "This analysis augments the strength of the conclusion that the events observed are due to top quark production and decay."

LBL physicists say that both experimental groups intend to continue searching for more examples of top quark production and decay until around the end of the year. At that time, the Tevatron will be shut down for system upgrades that will allow more rapid accumulation of data and a more complete understanding of top production and decay. LBL's CDF group has also been involved in measuring the mass of the W boson and has submitted a paper in which it reports the most precise measurements ever made of the mass of both the top quark and the W boson.

The W boson is one of two particles that carry the weak nuclear force. Physicists believe that precise measurements of the top quark mass, along with precise measurements of the W boson mass can provide information on the mass of the Higgs boson, a hypothetical, extremely massive boson arising in the theory of the electroweak force.

"The Higgs boson is now the only particle missing in the Standard Model," says Galtieri. "It is necessary to explain why the mediator of the electromagnetic force (the photon) has mass zero while the mediators of the weak force (the W and Z bosons) have masses in the 80 to 90 GeV range."

If the Higgs boson is not within reach of the Tevatron's energies (as is most likely the case), scientists will likely have to wait for the construction of the European Large Hadron Collider at CERN. For this reason, Madaras says of the top quark discovery, "I think that in particle physics, there will not be anything like this for another decade."

Current LBL staff members of the CDF collaboration include, in addition to Galtieri and Carithers: Willi Chinowski, Bob Ely, Kevin Einsweiler, Richard Kadel, Carl Haber, Young Kee Kim, Jeremy Lys, Manfred Paulini, Marjorie Shapiro, Hans Wenzel, and Weiming Yao. Among the participating students from the UC Berkeley were Bill Ashmanskas, Matt Austen, Mark Peters, and William Wester.

Current LBL staff members of the D0 collaboration include, in addition to Madaras and Trippe, Hiro Aihara, Lian-ping Chen, Al Clark, Orin Dahl, Roy Kerth, Fred Kral, Michael Levi, Stu Loken, Ed Oltman, Danilo Puseljic, Natalie Roe, Tony Spadafora, Lynn Stevenson, and Mark Strovink. Among the students participating were Justin Bendich, Azriel Goldschmidt, Peter Grundberg, Erich Varnes, and Patrick Virador.